26 U.S. Code § 6651 - Failure to file tax return or to pay tax
(a)Addition to the tax
In case of failure—
(1)to file any return required under authority of subchapter A of chapter 61 (other than part III thereof), subchapter A of chapter 51 (relating to distilled spirits, wines, and beer), or of subchapter A of chapter 52 (relating to tobacco, cigars, cigarettes, and cigarette papers and tubes), or of subchapter A of chapter 53 (relating to machine guns and certain other firearms), on the date prescribed therefor (determined with regard to any extension of time for filing), unless it is shown that such failure is due to reasonable cause and not due to willful neglect, there shall be added to the amount required to be shown as tax on such return 5 percent of the amount of such tax if the failure is for not more than 1 month, with an additional 5 percent for each additional month or fraction thereof during which such failure continues, not exceeding 25 percent in the aggregate;